Polarization preserving optical fiber and absolute single polarization optical fiber

ABSTRACT

A polarization-maintaining optical fiber and absolutely single-polarization optical fiber are provided which enable long distance transmission maintaining a polarization state of signal light. The polarization-maintaining optical fiber includes a core, photonic crystal cladding, and jacket. The photonic crystal cladding is divided into four segments by broken lines from the center to the periphery. Among a plurality of grating holes in a pair of segments opposed to each other with respect to the core, the diameter of grating holes close to the core is greater than the diameter of grating holes in another pair of segments opposed with each other, and than a grating constant.

TECHNICAL FIELD

The present invention relates to a transmission medium and opticaldevice, and more particularly to a polarization-maintaining opticalfiber and absolutely single-polarization optical fiber that are used foran optical communication network and optical signal processing to carryout transmission and multi/demultiplexing of light maintaining apolarization state.

BACKGROUND ART

A polarization-maintaining optical fiber is given a difference betweenpropagation constants of two polarization modes that have linearpolarization along two orthogonal principal axes in the core by makingthe modes of the fiber not be degenerated by applying anisotropy of thestress distribution in a single-mode optical fiber. Since the structureenables the distinction between the two polarization modes, when lightthat agrees with a particular polarization mode is launched to theoptical fiber, the light propagates through the optical fibermaintaining only that polarization mode.

As a typical polarization-maintaining optical fiber, a PANDA fiber isknown. The PANDA fiber, however, requires high technology that boresholes through two places in a base material (cladding) of the opticalfiber in extremely close proximity to the core region in the fabricationprocess, and that fills the holes with a stress-applying material toform the fiber. In particular, the process of squeezing thestress-applying material into the base material is a major factor ofreducing the productivity of the polarization-maintaining optical fiber.For this reason, the PANDA fiber usually costs 100 or more times higherthan an ordinary single-mode fiber. In addition, since the propagationconstant difference between the two orthogonal polarization modesresulting from the PANDA fiber structure is rather small, it isdifficult to reduce the crosstalk between the two modes to less than −30dB.

Thus, it is difficult for the PANDA fiber to transmit a signal pulsetrain over a long distance maintaining the single polarization.Accordingly, it is not used as a single polarization transmission path.Considering such difficulties in fabricating the PANDA fiber, opticalfibers with claddings with a variety of structures have been developedtoday.

FIG. 1 is a cross-sectional view showing a structure of a conventionalpolarization-maintaining optical fiber based on a photonic crystalstructure. The polarization-maintaining optical fiber comprises a coreregion 41, a photonic crystal cladding 42 and a jacket 43. In FIG. 1,the photonic crystal cladding 42 is divided into four segments 42 a, 42b, 42 c and 42 d by broken lines from the center to the periphery.

In the segments 42 a, 42 b, 42 c and 42 d, the grating constant Λ of thediffraction grating that consists of grating holes indicated by circlesin FIG. 1 is the same throughout the grating. However, the diameter d2of individual grating holes in the first opposed segments 42 a and 42 cis greater than the diameter d1 of individual grating holes in thesecond opposed segments 42 b and 42 d adjacent to the first opposedsegment (d2>d1). Such a structure can bring about the propagationconstant difference between x and y directions, thereby being able toimplement the polarization maintaining property.

FIG. 2 is a graph illustrating variations in the modal birefringencewhen varying the ratio of the diameters of the air holes of thepolarization-maintaining optical fiber. The detail of the calculation isdescribed in “Polarization maintaining holely optical fiber” (Kawanishiand Okamoto, 2000 Communications Society Conference No. B-10-153 of TheInstitute of Electronics, Information and Communication Engineers ofJapan).

The modal birefringence B is given by the following expression when thepropagation constants corresponding to the two perpendicularpolarization modes (HE11x mode and HE11y mode) in the fiber are βx andβy.B=(βx−βy)/k(K is a wave number)Here, the calculation is carried out using a finite element method.

It is clear from FIG. 2 that the modal birefringence B, a measure of thepolarization maintaining property, increases with an increase of theratio (d2/d1). In addition, the ratio (d2/d1) equal to or greater thantwo can implement the birefringence equal to or greater than that of theconventional PANDA polarization-maintaining optical fiber (about 5×10⁻⁴in PANDA). To increase (d2/d1), there is a method of increasing thediameter d2 or decreasing the diameter d1.

As for the polarization-maintaining optical fibers with such astructure, their prototypes and calculation examples are disclosed inthe following two documents.

-   -   (1) A. Ortigosa-Blanch, J. C. Knight, W. J. Wadsworth, J.        Arriaga, B. J. Mangan, T. A. Birks, P. St. Russell “Highly        birefringent photonic crystal fibers” Optics Letters, Vol. 25,        pp. 1325-1327 (2000); and    -   (2) S. B. Libori, J. Broeng, E. Knudsen, A. Bjarklev,        “High-birefringent photonic crystal fiber” OFC 2001, TuM2,        Anaheim (2001).

FIGS. 3 and 4 show cross-sectional structures of thepolarization-maintaining optical fiber described in the foregoingdocuments: FIG. 3 shows a picture of an actually fabricated device; andFIG. 4 shows a structure drawn according to calculation values.

In the examples as shown in FIGS. 3 and 4, the diameters d2 of all thegrating holes in the diffraction grating are less than the gratingconstant Λ. In addition, the diameter of the grating holes in a pair ofopposed segments with respect to the core region differs from thediameter of the grating holes in another pair of opposed segments,thereby achieving the polarization maintaining characteristic.

In the polarization-maintaining optical fiber as shown in FIGS. 3 and 4,the modal birefringences B at a wavelength 1550 nm (calculation values)are 2.8×10⁻³ and 1.5×10⁻³, respectively.

However, when the diameter d1 is less than the grating constant as inthe case of FIGS. 3 and 4, the optical confinement in the segments isweak, so that the light leaks from the core region to the segments,thereby the optical intensity distribution is distorted. In the worstcase, the majority of the optical intensities might present outside ofthe core region. As a result, the fiber is susceptible to a bendingloss, and becomes unusable as the fiber. Accordingly, it is impossibleto set the diameter dl below a certain value.

In addition, the example of FIG. 4 has a structure in which gratingholes with a greater diameter deviate from the surrounding diffractiongrating to the core region. However, it is difficult to fabricate such apolarization-maintaining optical fiber in practice.

As described above, the conventional polarization-maintaining opticalfiber has a problem of having difficulty in implementing the followingrequirements at the same time: maintaining the polarization state of thesignal light; carrying out long distance transmission; and fabricatingwith ease.

Furthermore, the conventional polarization-maintaining optical fiberallows the two orthogonal polarization modes to present within it. Thus,when the light travels a long distance through the fiber, slightcrosstalk arises between the two polarization modes even if thepolarization-maintaining optical fiber has the birefringence.Accordingly, it has a problem of inducing a perpendicular component atthe output due to the polarization crosstalk, even if polarization stateof the input light is set to one of the principal axes of the fiber. Infact, as for the PANDA fiber, the polarization crosstalk becomes aproblem when the propagation distance exceeds 20 km.

Therefore an object of the present invention is to provide apolarization-maintaining optical fiber and absolutelysingle-polarization optical fiber capable of implementing the longdistance transmission maintaining the polarization state of the opticalsignal.

Another object of the present invention to provide an absolutelysingle-polarization optical fiber enabling only one of the polarizationmodes to propagate through the fiber by providing a structure forabsorbing the other of the polarization modes.

DISCLOSURE OF THE INVENTION

According to the first aspect of the present invention, there isprovided a polarization-maintaining optical fiber including a core and acladding that is disposed about the core and includes grating holesarranged at a predetermined grating constant to confine light within thecore, the polarization-maintaining optical fiber is characterized inthat: a diameter of a pair of grating holes among the grating holes isgreater than a diameter of the remaining grating holes and than thepredetermined grating constant, the pair of grating holes being opposedwith respect to the core and disposed near the core.

The cladding may be a photonic crystal cladding.

The sum of the diameter of the pair of grating holes and the diameter ofthe remaining grating holes may be less than twice the predeterminedgrating constant.

Thus, it can increase the modal birefringence effectively.

According to the second aspect of the present invention, there isprovided an absolutely single-polarization optical fiber including acore and a cladding that is disposed about the core and includes gratingholes arranged at a predetermined grating constant to confine lightwithin the core, the absolutely single-polarization optical fiber ischaracterized in that: a diameter of a pair of grating holes among thegrating holes is greater than a diameter of the remaining grating holesand than the predetermined grating constant, the pair of grating holesbeing opposed with respect to the core and disposed near the core, andat least one of the pair of grating holes having its inside coated witha metal film.

According to the third aspect of the present invention, there isprovided an absolutely single-polarization optical fiber including acore and a cladding that is disposed about the core and includes gratingholes arranged at a predetermined grating constant to confine lightwithin the core, the absolutely single-polarization optical fiber ischaracterized in that:

-   -   a diameter of a pair of grating holes among the grating holes is        greater than a diameter of the remaining grating holes and than        the predetermined grating constant, the pair of grating holes        being opposed with respect to the core and disposed near the        core, and at least one of the pair of grating holes having its        inside covered with a multilayer composed of two types of media        that have different refractive indices and are stacked        alternately.

The cladding may be a photonic crystal cladding.

The sum of the diameter of the pair of grating holes and the diameter ofthe remaining grating holes may be less than twice the predeterminedgrating constant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a structure of a conventionalpolarization-maintaining optical fiber;

FIG. 2 is a graph illustrating relationships between the ratio ofdiameters of air holes and the modal birefringence B in the conventionalpolarization-maintaining optical fiber;

FIG. 3 is a cross-sectional view showing a structure of apolarization-maintaining optical fiber described in the document (1);

FIG. 4 is a cross-sectional view showing a structure of apolarization-maintaining optical fiber described in the document (2);

FIG. 5 is a cross-sectional view showing a structure of apolarization-maintaining optical fiber in accordance with the presentinvention;

FIG. 6 is a cross-sectional view showing a structure of apolarization-maintaining optical fiber in accordance with the presentinvention;

FIG. 7 is a graph illustrating calculation results of the modalbirefringence B versus wavelength when varying the diameters of thegrating holes near the core region;

FIG. 8 is a graph illustrating measurement results of the modalbirefringence B of a polarization-maintaining optical fiber that isactually fabricated in accordance with the present invention; and

FIG. 9 is a diagram showing part of the cross section of an absolutelysingle-polarization optical fiber in accordance with the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the invention will now be described withreference to the accompanying drawings to explain the present inventionin more detail.

FIG. 5 is a cross-sectional view showing a structure of apolarization-maintaining optical fiber in accordance with the presentinvention. The polarization-maintaining optical fiber comprises a coreregion 11, a photonic crystal cladding 12, and a jacket 13.

The photonic crystal cladding 12 includes grating holes disposed at apredetermined grating constant Λ to confine the light in the core, andconsists of four segments 12 a, 12 b, 12 c and 12 d divided by brokenlines extending from the center to periphery. Here, the diameter d2 ofthe grating holes 14 a and 14 c, which are disposed closest the coreregion 11 among a plurality of grating holes in a pair of segments 12 aand 12 c opposed to each other with respect to the core region 11, isgreater than the diameter d1 of the grating holes of another pair ofsegments 12 b and 12 d opposed to each other with respect to the coreregion 11. In addition, the diameter d2 of the grating holes in thesegments 12 a and 12 c is greater than the grating constant Λ.

Thus making the diameter d2 of the grating holes 14 a and 14 c closestto the core region 11 greater than the grating constant Λ enables anincrease of the modal birefringence B concerning the x and y directions.

To form the grating holes 14 a and 14 c with a diameter greater than thegrating constant Λ, it is necessary to determine the relationship withthe diameter of the other grating holes. In the example of FIG. 5, thefollowing relationship is satisfied.d 1+d 2<2ΛSatisfying the condition can maintain the grating constant Λ in theentire region, thereby enabling the diameter of the two grating holesclosest to the core region to be greater than the grating constant Λwithout changing the arrangement (center position) of the grating.

Such a polarization-maintaining optical fiber can be fabricated by aknown fabrication method of the photonic crystalpolarization-maintaining optical fiber such as disclosed in “Low-loss,2-km-long photonic crystal fiber with zero GVD in the near IR suitablefor picosecond pulse propagation at the 800 nm band”, (H. Kubota, K.Suzuki, S. Kawanishi, M. Nakazawa, M. Tanaka, andM. Fujita, Tech. Digestof Conference on Lasers and Electro-optics (CLEO), CPD3, 2001).

FIG. 6 shows a preferred embodiment in accordance with the presentinvention. It employs a photonic crystal fiber 1 including in an opticaltransmission region 3 a Ge-doped elliptical core 2 that has a refractiveindex higher than that of its surroundings, and has a cross section witha flat shape, which causes the polarization maintaining property. Inaddition, since the slow axis of the optical transmission region 3 andthat of the core 2 point the same direction, the polarization planemaintained in the optical transmission region 3 and the polarizationplane maintained in the core 2 agree with each other. Accordingly, thepolarization maintaining property of the two are superimposed, therebyproviding a large polarization maintaining characteristics that cannotbe obtained by one of the two.

The flat shape of the optical transmission region 3 and core 2 has aslow axis and a fast axis orthogonal to each other. It is preferablethat the slow axis be 1.3-5.0 times longer than the fast axis because itcan increase the polarization maintaining characteristic. The flat shapecan be mentioned as a narrow shape.

FIG. 7 is a graph illustrating calculation results of the modalbirefringence B for the wavelength as to the polarization-maintainingoptical fiber with the grating holes closest to the core region, whichare different in diameter from the other grating holes. In the exampleof FIG. 7, the grating constant Λ is set at 4 μm, and the diameter d1 ofthe grating holes in the segments 12 b and 12 d is set at 1.9 μm. Thecalculation is carried out with varying the diameter d2 as 3.6 μm, 4.0μm, 4.4 μm and 4.7 μm. The calculation results are shown by curves 201,202, 203 and 204. It is seen from this graph that the modalbirefringence B increases with an increase in the diameter d2 of thegrating holes near the core in the segments 12 a and 12 c.

FIG. 8 is a graph illustrating measured results of the modalbirefringence B for the wavelength of a polarization-maintaining opticalfiber in accordance with the present invention. The measurement of FIG.8 was carried out for a sample polarization-maintaining optical fiber inwhich the grating constant Λ is 4 μm, the diameter d1 of the gratingholes in the segments 12 b and 12 d is 1.9 μm, and the diameter d2 ofthe grating holes near the core in the segments 12 a and 12 c is 4.7 μm.

As is clear from the comparison of FIGS. 7 and 8, the experimentalresults are in good agreement with the calculated values. In addition,it is seen that the modal birefringence B at the wavelength 1550 nm is1.4×10⁻³, which is greater than that of the conventional technique.

Although the embodiments in accordance with the present invention aredescribed above, the present invention is not limited to the foregoingembodiments, but is applicable to other variations.

For example, one or both of the grating holes 14 a and 14 c near thecore 11 in the segments 12 a and 12 c of the grating holes as shown inFIG. 5 can have the inside coated with a metal such as aluminum or gold.Fixing the metal inside the grating holes 14 a and 14 c makes itpossible to absorb the polarization with an electric field componentperpendicular to the metal surface, thereby enabling only thepolarization with the electric field component parallel to the metalsurface to propagate through the optical fiber.

Alternatively, as shown in FIG. 9, a multilayer, which consists of twomedia 801 and 802 with different refractive indices stacked alternately,can be fixed to the inside of the grating holes 14 a and 14 c near thecore 11. With such a structure, it is possible to increase theabsorption loss of only one of the polarization modes because thereflectance and transmittance of the light vary depending on theincident optical polarization direction. Thus,only one of thepolarization modes propagates through the optical fiber in accordancewith the present invention.

Although FIG. 9 shows an example in which both the grating holes 14 aand 14 c have their inside covered with the multilayer, this is notessential. For example, only one of the grating holes 14 a and 14 c canhave its inside coated with the multilayer.

In this case, the optical fiber in accordance with the present inventionbecomes an absolutely single-polarization optical fiber that cantransmit only one of the polarization modes.

Industrial Applicability

As described above, according to the present invention, the photoniccrystal structure near the core is provided with a very large modalbirefringence, which enables the suppression of the polarizationcrosstalk, thereby being able to stabilize the signal light. As aresult, the present invention can implement a long distance transmissiongreater than the conventional polarization-maintaining optical fibermaintaining a signal optical polarization state.

In addition, employing the photonic crystal cladding including thediffraction grating with a specified grating constant makes it possibleto fabricate an optical fiber with a large modal birefringence veryeasily.

Furthermore, providing the structure for absorbing one of thepolarization modes in accordance with the present invention enables onlyone polarization mode to propagate through the optical fiber over a longdistance.

1. A polarization-maintaining optical fiber including a core and acladding that is disposed about the core and includes grating holesarranged at a predetermined grating constant to confine light within thecore, said polarization-maintaining optical fiber is characterized inthat: a diameter of a pair of grating holes among the grating holes isgreater than a diameter of the remaining grating holes and than thepredetermined grating constant, said pair of grating holes being opposedwith respect to the core and disposed near the core.
 2. Thepolarization-maintaining optical fiber as claimed in claim 1, whereinsaid cladding is a photonic crystal cladding.
 3. Thepolarization-maintaining optical fiber as claimed in claim 1, wherein asum of the diameter of said pair of grating holes and the diameter ofthe remaining grating holes is less than twice the predetermined gratingconstant.
 4. An absolutely single-polarization optical fiber including acore and a cladding that is disposed about the core and includes gratingholes arranged at a predetermined grating constant to confine lightwithin the core, said absolutely single-polarization optical fiber ischaracterized in that: a diameter of a pair of grating holes among thegrating holes is greater than a diameter of the remaining grating holesand than the predetermined grating constant, said pair of grating holesbeing opposed with respect to the core and disposed near the core, andat least one of said pair of grating holes having its inside coated witha metal film.
 5. The absolutely single-polarization optical fiber asclaimed in claim 4, wherein said cladding is a photonic crystalcladding.
 6. The absolutely single-polarization optical fiber as claimedin claim 4, wherein a sum of the diameter of said pair of grating holesand the diameter of the remaining grating holes is less than twice thepredetermined grating constant.
 7. An absolutely single-polarizationoptical fiber including a core and a cladding that is disposed about thecore and includes grating holes arranged at a predetermined gratingconstant to confine light within the core, said absolutelysingle-polarization optical fiber is characterized in that: a diameterof a pair of grating holes among the grating holes is greater than adiameter of the remaining grating holes and than the predeterminedgrating constant, said pair of grating holes being opposed with respectto the core and disposed near the core, and at least one of said pair ofgrating holes having its inside covered with a multilayer composed oftwo types of media that have different refractive indices and arestacked alternatively.
 8. The absolutely single-polarization opticalfiber as claimed in claim 7, wherein said cladding is a photonic crystalcladding.
 9. The absolutely single-polarization optical fiber as claimedin claim 7, wherein a sum of the diameter of said pair of grating holesand the diameter of the remaining grating holes is less than twice thepredetermined grating constant.
 10. The polarization-maintaining opticalfiber as claimed in claim 1, wherein the core has an elliptical Ge-dopedarea.